Dielectric materials have traditionally been used for insulating conductor patterns on ceramic substrates. The principal properties required for many such applications have been the electrical properties such as insulation resistance and dissipation factor of the dielectric, as measured with the conductor of choice. Many dielectric materials in use for many years have been non-hermetic type dielectrics; that is, they may contain a significant micro-porosity while meeting all other needs for making crossover and low-layer count multilayers. Typically the choice of thick film conductor (Ag, Cu, Ni or Au) dictated the level of reliability that the circuit may possess due to the tendency of the conductor to migrate and short circuit when exposed to humidity and electric field in use. Noble metals like gold are the most reliable, while base metals like silver are the least reliable. The cost was approximately proportional to the need for reliability.
The trend of the electronic industry has been toward higher circuit (closer conductor lines) and higher reliability requirements, while at the same time intense pressure for lower costs of manufacture has driven the circuit manufacturer to consider the use of less costly metals such as silver and copper. The industry has been moving toward the use of multilayer circuits. The use of double sided boards with crossovers and finer conductor line geometries, however, has been the first stage in this trend.
To permit the manufacture of reliable circuits, utilizing base metal conductors, dielectric materials which resist migration of conductive materials on the surface or through the bulk of the dielectric are necessary. Since moisture plays a large role in the migration of conductive phases, hermetic dielectric materials are essential. Once the conductor line is sealed between layers of hermetic dielectric, conductor migration ceases to be a serious reliability risk.
In addition, however, to the requirements for sealed conductor structures, the thermal expansion of the dielectric must be carefully matched to the substrate material. The dielectric must also be capable of undergoing multiple reheat cycles to the firing temperature (usually 850.degree. C.) without continued migration of the conductive flux through the dielectric. In complex circuits, a large number of refires are frequently necessary. Large numbers of refires and the necessity to reduce cost have made the possiblity of co-firing pairs of layers in a multilayer circuit quite desirable.
Most multilayer dielectrics are mixtures of glass and refractory ceramic oxides. They tend to be porous because they are normally formulated with organic binder materials which evolve decomposition gases during thermal processing of applied layers of dielectric, thus leaving open passages in the formed dielectric layers through which these gases have escaped. For this reason multiple print and firing of dielectric layers is commonly performed to close connected porosity.
Many available dielectric materials, after repeated firing of circuits, develop leaky, soft shorts or some hard shorted paths when the dielectric layers are degraded by flux penetration from the base metal conductor. These flux materials after repeated firing chemically reduce to their respective metals/alloys yielding a variety of electrical failure possibilities. The flux materials can become conductive by reduction reactions triggered by binder exhaust gases and residual carbon in the materials.